Backscattering Ambient ISM Band Signals

ABSTRACT

A backscatter tag device includes, in part, a receiver configured to receive a packet conforming to a communication protocol defining a multitude of codewords, a codeword translator configured to translate at least a first subset of the multitude of codewords disposed in the packet to a second multitude of codewords defined by the protocol in response to a data the backscatter tag is invoked to transmit, and a transmitter configured to transmit the packet supplied by the codeword translator at a frequency different than the first frequency at which the packer is received. The communication protocol may optionally be the 802.11 g/n, ZigBee or the Bluetooth communication protocol.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application claims benefit under 35 USC 119(e) of U.S.Application Ser. No. 62/412,712, filed Oct. 25, 2016, entitled“Freerider: Backscattering Ambient ISM Band Signals”, the content ofwhich is incorporated herein by reference in its entirety.

The present invention is related to application Ser. No. 15/676,474,filed Aug. 14, 2017, the content of which is incorporated herein byreference in its entirety.

FIELD OF THE INVENTION

The present invention relates to communication systems and methods, andmore particularly to a low power WiFi backscattering communicationsystem and method.

BACKGROUND OF THE INVENTION

Backscatter communication has attracted interest for applications suchas implantable sensors, wearables, and smart home sensing because of itsability to offer low power connectivity to these sensors. Suchapplications have severe power constraints. Implantable sensors forexample have to last for years, while even more traditional smart homemonitoring applications may benefit from sensors and actuators that canlast several years. Backscatter communication can satisfy theconnectivity requirements while consuming such low power as to beenergized by harvesting energy, or with batteries that could lastseveral years.

Current backscatter systems require specialized hardware to generate theexcitation RF signals that backscatter radios can reflect, as well as todecode the backscattered signals. Recent research such as Wi-Fibackscatter to BackFi and passive WiFi have reduced the need forspecialized hardware. Passive WiFi for example can decode using standardWiFi radios, however it still requires a dedicated continuous wavesignal generator as the excitation RF signal source. BackFi needs aproprietary full duplex hardware add-on to WiFi radios to enablebackscatter communication. Consequently, a need continues to exist for abackscatter system that can be deployed using commodity devices such asaccess points, smartphones, watches and tablets.

BRIEF DESCRIPTION OF THE INVENTION

FIG. 1 is a simplified view of a backscatter communication system 100,in accordance with one embodiment of the present invention.

FIG. 2 shows a number of OFDM symbols modulated on differentsubcarriers, as known in the prior art.

FIG. 3 shows the durations of a number of packets collected on achannel.

FIG. 4 shows the rate of decoding success as a function of distance, inaccordance with one exemplary embodiment of the present invention.

FIG. 5 is a simplified high-level block diagram of a backscatter tag, inaccordance with one embodiment of the present invention.

FIG. 6 shows various block diagrams of an 802.11 g/n transmission andreception blocks

FIG. 7 is a more detailed view of the scrambler shown in FIG. 6.

FIG. 8 shows the frequency spectrum of a backscattered Bluetooth signal.

FIG. 9A shows an experimental setup for testing a backscatter tagdeployed in a line-of-sight, in accordance with one embodiment of thepresent invention.

FIG. 9B shows an experimental setup for testing a backscatter deployedin a non-line-of sight setup, in accordance with one embodiment of thepresent invention.

FIG. 10A shows the throughput of a tag, in accordance with oneembodiment of the present invention, as a function of distance betweenthe tag and the receiver in a LOS deployment.

FIG. 10B shows the bit-error rate of a tag, in accordance with oneembodiment of the present invention, as a function of distance betweenthe tag and the receiver in a LOS deployment.

FIG. 10C shows the received signal strength indicator of a tag, inaccordance with one embodiment of the present invention, as a functionof distance between the tag and the receiver in a LOS deployment.

FIG. 11A shows the throughput of a tag, in accordance with oneembodiment of the present invention, as a function of distance betweenthe tag and the receiver in an NLOS deployment.

FIG. 11B shows the bit-error rate of a tag, in accordance with oneembodiment of the present invention, as a function of distance betweenthe tag and the receiver in an NLOS deployment.

FIG. 11C shows the received signal strength indicator of a tag, inaccordance with one embodiment of the present invention, as a functionof distance between the tag and the receiver in an NLOS deployment.

FIG. 12A shows the throughput of a tag, in accordance with oneembodiment of the present invention, as a function of distance betweenthe tag and a ZigBee receiver.

FIG. 12B shows the bit-error rate of a tag, in accordance with oneembodiment of the present invention, as a function of distance betweenthe tag and a ZigBee receiver.

FIG. 12C shows the received signal strength indicator of a tag, inaccordance with one embodiment of the present invention, as a functionof distance between the tag and a ZigBee receiver.

FIG. 13A shows the throughput of a tag, in accordance with oneembodiment of the present invention, as a function of distance betweenthe tag and a Bluetooth receiver.

FIG. 13B shows the bit-error rate of a tag, in accordance with oneembodiment of the present invention, as a function of distance betweenthe tag and a Bluetooth receiver.

FIG. 13C shows the received signal strength indicator of a tag, inaccordance with one embodiment of the present invention, as a functionof distance between the tag and a Bluetooth receiver.

FIG. 14 shows the effect of the distance between the tag, in accordancewith embodiments of the present, and the transmitter for WiFi 802.11g/n, ZigbBee and Bluetooth communications protocols, in accordance withone embodiment of the present invention.

FIG. 15 shows the throughput of a WiFi 802.11 g/n when the backscattertag is either present or absent, in accordance with one embodiment ofthe present invention.

FIG. 16A shows the backscatter tag throughput when an 802.11g/n WiFi isused as the excitation signal for the backscatter tag, in accordancewith one embodiment of the present invention.

FIG. 16B shows the backscatter tag throughput when a ZigBee is used asthe excitation signal for the backscatter tag with the tag, inaccordance with one embodiment of the present invention.

FIG. 16C shows the backscatter tag throughput when a Bluetooth is usedas the excitation signal for the backscatter tag with the tag, inaccordance with one embodiment of the present invention.

FIG. 17A shows the aggregated throughput when respectively 4, 8, 12, 16,and 20 tags are positioned in the path of the transmitter, in accordancewith one embodiment of the present invention.

FIG. 17B shows the Jain's fairness index when respectively 4, 8, 12, 16,and 20 tags are positioned in the path of the transmitter, in accordancewith one embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention provide a system and method ofcommunication that is complaint with an existing communicationsprotocol, such as WiFi 802.11g/n, Bluetooth, and ZigBee, bybackscattering another compliant packet and modulating its data on theresulting packet by codeword translation. According to some embodiments,applications can be built on existing wireless devices carrying suchpackets. A low-power backscatter communications system (hereinafteralternatively referred to as backscatter tag, or tag) is configured, inpart, to receive a valid codeword disposed in the transmitted, forexample, 802.11 g/n packet and translate it to a different validcodeword from, for example, the 802.11 g/n codebook. The specifictranslation encodes the bit that the backscatter tag seeks to transmit.The backscattered packet is therefore like any other, for example,802.11g/n packet, albeit with a sequence of translated codewordsdepending on the data that backscatter tag seeks to communicate.Consequently it can be decoded by any standard 802.11g/n, WiFi,Bluetooth, and ZigBee receiver. The following description of theembodiments of the present invention is provided with reference to WiFi802.11g/n, Bluetooth, and ZigBee communications protocol or standards.It is understood however that embodiments of the present invention areequally applicable to many other communication protocols.

A backscatter communication system, in accordance with embodiments ofthe present invention, may use commodity radios by using codewordtranslation. As is known, any wireless signal on the ISM band isgenerated using a set of known codewords from a fixed codebook. Forexample, Bluetooth uses FSK modulation and has two codewords in itscodebook: it transmits a tone at one frequency to send a one, and adifferent frequency to send a zero. Similarly, WiFi and ZigBee also havefinite sets of codewords that vary in combinations of phase, amplitudeor frequency.

To perform codeword translation, a tag transforms (or translates) theongoing excitation signal's codeword into another valid codeword in thesame codebook during backscattering. This is achieved by modifying oneor more of the amplitude, phase, or frequency of the excitation signal.The specific translation depends on the data that the tag seeks tocommunicate and the type of the excitation signal. Because the codewordin the backscattered signal is a valid codeword from the same codebookas the original excitation signal, a commodity radio may be used toreceive the backscattered signal.

FIG. 1 is a simplified view of a backscatter communication system 100,in accordance with one embodiment of the present invention. An ISM bandradio, which may be a WiFi, Bluetooth or ZigBee transmitter, transmitsdata in the form of packets 15 during normal operation to receiver 30,which may be a commodity receiver, such as WiFi, Bluetooth or ZigBee. Aninternet-of-things (IoT) device 20 (which is also referred to herein asa tag) also receives packets 15, implements codeword translation toembed the information that tag 20 seeks to transmit as described furtherbelow, and backscatters the codeword translated packet 25 to receiver40, which may also be a commodity receiver, such as WiFi, Bluetooth orZigBee. The backscattered signal (alternatively referred to herein aspacket) 25 is frequency shifted to another channel. Accordingly,receiver 30 receives and decodes the originally transmitted packets 15and receiver 40 receives and decodes the backscattered packets 25.Decoder 50 is configured to compare the packets received by receivers 30and 40 thereby to retrieve the tag data embedded by tag 20.

Embodiments of the present invention support multiple tags on the samewireless channel by leveraging packet length modulation to transmitnecessary information to the tags for coordination. To achieve this, thelength of the excitation packet 15 is used to encode 0s and 1s, whichcan be arranged to form messages to the tags that implement abackscatter MAC protocol. The protocol thus sends control messages tothe tags so that the tags can coordinate their transmissions to avoidcollisions.

Embodiments of the present invention achieve, among other things, thefollowing objectives. One embodiment decodes backscattered OFDM WiFisignals from 42 m in a line-of-sight (LOS) deployment, and 22 m in anon-line-of-sight (NLOS) deployment. One embodiment achieves athroughput of nearly 60 kbps from a backscattered OFDM WiFi signal whena LOS receiver is 18 m or closer. For farther distances, in one example,an average of 32 kbps (LOS) and 20 kbps (NLOS) is achieved. Oneembodiment backscatters ZigBee signals from up to, for example, 22 m,achieving 15 kbps. One embodiment backscatters Bluetooth signals up to,for example, 12 m, achieving 55 kbps. A backscatter tag, in accordancewith embodiments of the present invention may coexists with WiFinetworks independent of the type of excitation signal the tag isbackscattering. Furthermore, in one experimental setup, up to 20backscatter tags were shown to operate effectively while communicatingsuccessfully in a MAC scheme and maintaining uplink fairness.

When a tag, in accordance with embodiments of the present invention,backscatters an excitation signal, the tag may modify one or more of thesignal's amplitude, phase, or frequency. Such modification is shownbelow where S(t) represents the excitation signal, T(t) represents thetag signal, and B(t) represents the backscattered signal. Thebackscattered signal B(t) is the time domain product between theexcitation signal S(t) and the tag signal T(t). Therefore, a tag maychange its signal T(t) to modify the amplitude, phase, and frequency ofthe backscattered signal B(t). Signals S(t), T(t), and B(t) may berepresented as shown below:

$\begin{matrix}{{{S(t)} = {A_{s}e^{j{({{2\pi \; f_{s}t} + \theta_{s}})}}}}{{T(t)} = {A_{t}e^{j{({{2\overset{.}{\pi}\; f_{t}t} + \theta_{t}})}}}}\begin{matrix}{{B(t)} = {{S(t)}{T(t)}}} \\{= {A_{s}e^{j{({{2\pi \; f_{s}t} + \theta_{s}})}}A_{t}e^{j{({{2\pi \; f_{t}t} + \theta_{t}})}}}} \\{= {A_{s}A_{t}e^{j{({{2{\pi {({f_{s} + f_{t}})}}t} + {({\theta_{s} + \theta_{t}})}})}}}}\end{matrix}} & (1)\end{matrix}$

A tag, in accordance with embodiments of the present invention, isconfigured to modify the amplitude of the backscattered signal by tuningthe terminating impedance of the tag antenna. The backscattered signalB(t) strength is defined by:

$\Gamma = \frac{Z_{T} - Z_{A}^{*}}{Z_{A} + Z_{T}}$

In the above expression, Z_(A) represents the tag antenna impedance andZ_(T) represents the impedance across tag antenna terminals A more exactrelationship between the backscattered signal strength and F may be seenin “Hybrid analog-digital backscatter: A new approach for battery-freesensing in RFID (RFID)”, authored by Vamsi Talla and Joshua R Smith,IEEE International Conference on IEEE, 2013, pp. 74-81.

In a conventional backscatter system, a tag switches between Z_(T) ₁=Z_(A) and Z_(T) ₂ =0 to encode information. Therefore, two levels ofamplitude are seen on a backscattered signal. Instead of switchingbetween two impedances, as is done conventionally for creating theanalog backscatter signal, a tag, in accordance with embodiments of thepresent invention varies across a multitude of impedances to fine tunethe amplitude of the backscattered signal.

A tag, in accordance with one embodiment of the present invention,changes the phase of the backscattered signal by delaying the tagsignal. In order to introduce an additional phase offset Δθ at the tag,the tag signal is delayed by

$\frac{\Delta \; \theta}{2\pi \; f_{t}}.$

The phase onset Δθ introduced at the tag leads to a phase offset on thebackscattered signal. To change the frequency of the backscatteredsignal, the tag changes the toggling frequency of its RF transistor.Therefore, a tag, in accordance with embodiments of the presentinvention, is configured to modify the amplitude, phase, and frequencyof the backscattered signal, thereby to enable backscatter communicationbetween commodity radios.

Codeword Translation

To communicate with a commodity radio, a backscatter tag, in accordancewith one embodiment of the present invention, performs codewordtranslation, as described further

A codeword C_(i) is defined herein as a signal symbol on the physicallayer that represents specific data transmitted. For example, Bluetoothuses binary FSK modulation to embed information. Therefore, it only usestwo codewords, C₁=e^(j2πf) ¹ ^(t) and C₂=e^(j2πf) ² ^(t) to representdata one and data zero respectively.

A codebook B is the set of valid codewords used by a radio. The codebookassociated with Bluetooth is B={C₁, C₂} because only two codewords areused in the Bluetooth standard. Similarly, WiFi 802.11g/n standard usesa codebook B={C₁, C₂ . . . C_(n)}, where C_(i) wherein i is an indexranging from 1 to n, and is an OFDM symbol. As is well known, the WiFi,ZigBee and Bluetooth use different sets of codewords and codebooks.

Different codewords in the same codebook are related to each other by ashift in phase, amplitude, frequency or a combination thereof. Forexample, the codeword C₁ used by the Bluetooth standard only differsfrom C₂ in the Bluetooth standard in the frequency domain, with afrequency difference of f₂−f₁.

Codeword translation is the act of transformation of a valid codewordC_(i) to another valid codeword C_(j) where both codewords belong to thesame codebook, meaning C_(i) ∈ B and C_(j) ∈ B. A backscatter tag, inaccordance with embodiments of the present invention, performs suchtranslation in compliance with WiFi, ZigBee, and Bluetooth standards,while consuming a relatively small amount of power. Because thetransformed/translated codeword remains a valid codeword in the samecodebook, a commodity WiFi, ZigBee, or Bluetooth radio may be used todecode the backscatter signal. The tag data is encoded by the specificcodeword translations, as described further below.

An example of codeword translation performed by a tag, in accordancewith embodiments of the present invention is shown in expression (2)below, where the codeword of the excitation signal is C_(i). To encode aone, the tag translates the codeword C_(i) to C_(j) before transmission.To encode a zero, the tag leaves the codeword untranslated, therefore,the backscatter signal has the same codeword as the excitation signal.

$\begin{matrix}{{{backscatter}\mspace{14mu} {codeword}} = \left\{ \begin{matrix}C_{j} & {{Tag}\mspace{14mu} {data}\mspace{14mu} {one}} \\C_{i} & {{Tag}\mspace{14mu} {data}\mspace{14mu} {zero}}\end{matrix} \right.} & (2)\end{matrix}$

By using codeword translation, the tag decodes the backscatter signalusing commodity WiFi, ZigBee, or Bluetooth radios to extract the tagdata. Table I below shows the logic table for decoding a backscattersignal.

TABLE I decoded codeword excitation signal codeword tag bit C₂ C₁ 1 C₁C₂ 1 C₁ C₁ 0 C₂ C₂ 0

As is seen from Table I, the tag bits are the XOR function of thebackscattered codeword and the original codeword. Therefore, the tagdata may be extracted by computing the XOR of the original excitationbitstream and the backscatter bitstream.

As was described above, a tag performs codeword translation by modifyingthe amplitude, phase, or frequency of the excitation signal. Suchmodification transforms the excitation codeword from C_(i) to C_(j) inthe backscattered signal. A tag performing codeword translation isfrequency agnostic, and thus applies the same modification on signalsacross all frequencies. This does not pose any problems for standardsthat use a single carrier wave, such as Bluetooth, ZigBee, and 802.11bstandards. However, because an OFDM signal associated with the 802.11nstandard uses multiple subcarriers, the above codeword translation cancause problems. When a tag changes the amplitude of a signal onsubcarrier it will introduce the same amplitude modification on anothersubcarrier m. However, the modified signal on subcarrier m may not be avalid codeword.

An example of this is shown in FIG. 2 where the data modulated onsubcarrier i is 1000 while the data modulated on subcarrier m is 0101.When the tag transforms the signal on subcarrier i from 1000 to 1101 byreducing the signal amplitude, the tag applies the same operation onsubcarrier m, reducing the strength of the signal that represents 0101.As a result, the tag creates an invalid codeword on subcarrier m.Therefore, when a tag does codeword translation, it looks for a signalcharacteristics, such as amplitude, phase, or frequency, to performsignal modification/translation such that the modified signal remains avalid codeword.

Backscatter of OFDM Symbols

Equation 3 below shows an OFDM modulated signal where {X_(k)} are thedata symbols modulated on subcarriers, N is the number of sub-carriers,and T is the OFDM symbol time. For 802.11g/n standard, an OFDM symbollasts for 4 μs and contains 64 subcarriers. The data symbols {X_(K)} aregenerated using BPSK, QPSK, 16-QAM, or 64-QAM modulation depending onthe WiFi standard bit rate.

$\begin{matrix}{{S(t)} = {\sum\limits_{k = 0}^{N - 1}\; {X_{k}e^{\frac{2\pi \; {kt}}{T}}}}} & (3)\end{matrix}$

When backscattering an OFDM symbol, a tag, in accordance with oneembodiment of the present invention, does not modify the amplitude orfrequency of the excitation OFDM signal because such modificationcreates an invalid codeword in the backscattered signal. Therefore, thetag modifies only the phase of the backscattered signal. A binaryexample is shown in equation 4 below. The tag introduces a phase offsetΔθ to transmit a data one. It introduces no offset to transmit a datazero. The value of Δθ depends on the tag bit rate. For example, if thetag transmits a lower data rate, it uses the binary scheme where Δθ is180°. If the tag decides to transmit at higher data rate, it may chooseΔθ as 90° and use equation 5, shown below, to encode its information.

$\begin{matrix}{{B(t)} = \left\{ \begin{matrix}{{S(t)}e^{j\; 0}} & {{Tag}\mspace{14mu} {data}\mspace{14mu} 0} \\{{S(t)}e^{j\; \Delta \; \theta}} & {{Tag}\mspace{14mu} {data}\mspace{20mu} 1}\end{matrix} \right.} & (4) \\{{B(t)} = \left\{ \begin{matrix}{{S(t)}e^{j\; 0}} & {{Tag}\mspace{14mu} {data}\mspace{14mu} 00} \\{{S(t)}e^{j\; \Delta \; \theta}} & {{Tag}\mspace{14mu} {data}\mspace{14mu} 01} \\{{S(t)}e^{j\; 2\Delta \; \theta}} & {{Tag}\mspace{14mu} {data}\mspace{14mu} 10} \\{{S(t)}e^{j\; 3\Delta \; \theta}} & {{Tag}\mspace{14mu} {data}\mspace{14mu} 11}\end{matrix} \right.} & (5)\end{matrix}$

Backscatter with ZigBee

A ZigBee radio uses Offset QPSK (OQPSK) modulation. Similar to QPSKmodulation, the data is encoded in the phase of the transmitted signal.Therefore, a tag, in accordance with embodiments of the presentinvention, embeds data in an OQPSK signal by modifying the phase duringreflection. When the tag transmits a data one, it introduces a Δθ phaseoffset on the reflected signal. When the tag transmits a data zero, itdoes not change the phase. The formula for embedding tag bits in ZigBeeis the same for an 802.11g/n WiFi standards shown in equations 4 andequation 5 above.

Backscatter with Bluetooth.

A Bluetooth radio modulates information by changing the carrier signalfrequency between two frequencies f₁ and f₀ depending on the codewordtransmitted. To transmit a data one, the radio sends a sine wave withfrequency f₁. To transmit a data zero, the radio sends a sine wave withfrequency f₀. A tag, in accordance with the present invention, uses theformula shown in expression (6) below to embed its information. Whentransmitting data one, the tag generates an additional frequency offsetΔf in the backscattered signal by toggling its RF transistor atfrequency Δf. When transmitting data zero, the tag does not generate theadditional frequency offset. If we select Δf carefully, we can ensurethat B(t) is still a valid Bluetooth signal and can be decoded by acommercial Bluetooth radio

$\begin{matrix}{{B(t)} = {{{S(t)}{T(t)}} = \left\{ \begin{matrix}{{S(t)}e^{j{({2{\pi\Delta}\; f\; t})}}} & {{tag}\mspace{14mu} {data}\mspace{14mu} {one}} \\{S(t)} & {{tag}\mspace{14mu} {data}\mspace{14mu} {zero}}\end{matrix} \right.}} & (6)\end{matrix}$

One possible option to ensure that the B(t) remains a valid Bluetoothsignal is to select Δf to be defined by |f₁−f₀|. Assume that theBluetooth radio transmits a data one with frequency f₁. For the tag totransmit data one, it shift the signal by Δf so that the backscatteredcodeword ise^(j(2πf) ⁰ ^(t+θ) ^(S) ⁾. This is a valid Bluetooth FSK codeword because it is a sine wave with frequency f₀. However, a commercialBluetooth radio will decode it as a zero rather than a one. Conversely,to encode a data zero the tag does not frequency-shift the Bluetoothsignal. The case is symmetric when the Bluetooth radio transmits datazero with frequency f₀ instead. Therefore, to transmit a data one, atag, in accordance with one embodiment of the present invention,transforms a Bluetooth codeword with frequency f₁ to a backscatteredcodeword with frequency f₀, and transforms a Bluetooth codeword withfrequency f₀ to a backscattered codeword with frequency f₁. To transmita data zero, the tag generates a backscattered codeword with the samefrequency as the original Bluetooth codeword. Therefore, by selecting Δfas described above, a tag, in accordance with one embodiment of thepresent invention, generates a backscattered signal that is a validBluetooth signal while embedding the data it seeks to transmit.

Avoiding Interference from Active Radios

If a tag transmits a backscatter signal to a receiver, the receiver maysee severe interference from the excitation signal because both thebackscattered signal and the excitation signal share the same channel.To avoid such interference, a tag, in accordance with embodiments of thepresent invention, shifts the frequency of the backscatter signal toensure that it occupies a frequency channel different from the oneoccupied by the excitation signal. Such frequency shifting techniquesare described, for example, in a paper entitled “Enabling practicalbackscatter communication for on-body sensors”, authored by PengyuZhang, Mohammad Rostami, Pan Hu, and Deepak Ganesan, Proceedings of the2016 conference on ACM SIGCOMM 2016, pp. 370-383, or in a paper entitled“Inter-Technology Backscatter: Towards Internet Connectivity forImplanted Devices”, authored by Vikram Iyer, Vamsi Talla, Bryce Kellogg,Shyamnath Gollakota, and Joshua Smith, Proceedings of the 2016conference on ACM SIGCOMM 2016, pp. 356-369.

Such frequency shifting may be achieved, for example, by toggling the RFtransistor at the desired frequency offset. For example, if to shift thebackscattered signal 20 MHz away from the excitation signal, the RFtransistor is toggled at 20 MHz. In one example, when backscattering aWiFi signal, the tag shifts the frequency such that the backscatteredsignal is tuned to, for example, channel 13, which is the least usedchannel in the 2.4 GHz ISM band. Such channel allocation reducesinterference to and from other active radios. When backscatteringBluetooth or ZigBee, the tag shifts the frequency of the backscattersignals so that they are tuned to channels close to 2.48 GHz becausethese channels experience less interference from the WiFi signal.

MAC Protocol

To facilitate effective sharing of the wireless medium between multipletags, a media access (MAC) scheme is developed in accordance withembodiments of the present invention. The MAC protocol serves twopurposes, namely it informs the tag what signals to backscatter with,and further it provides support for multiple tags, as described furtherbelow.

Coordinating Tags

Determining when to backscatter is important. If the incorrect signal isbackscattered, data cannot be recovered. The tags need a way todistinguish when to start backscattering signals. To ensure that the tagstarts to backscatter at the appropriate time, the transmitter (e.g.,transmitter 10 in FIG. 1) sends a preamble containing a predeterminedsequence of 0s and 1s, described further below. The tag maintains acircular buffer of received bits. If the beginning of the buffer matchesthe preamble, the tag knows that the buffer contains backscatter datainitiated by a command from the transmitter and not random packets.

Communicating with Multiple Tags

Because a tag does not have sufficient power to perform carrier sensing,a random access scheme based on Framed Slotted Aloha protocol is used.In accordance with this protocol, the transmitter acts as a centralcoordinator, in the same manner as described in the publication “Anempirical study of UHF RFID performance” by Michael Buettner and DavidWetherall, Proceedings of the 14th ACM international conference onMobile computing and networking, 2008, pp. 223-234. Communication iscarried out in rounds with a fixed number of slots per round. In eachround, the tags choose a random slot to transmit. If two tags choose thesame slot, collision occurs and data is not successfully transmitted. Atthe end of a round, the transmitter processes data from the tags andadjusts the number of slots before proceeding to the next round.

Compared to a stochastically allocated time-division scheme, randomaccess allows the number of tags to grow and shrink without a specificassociation process. The number of slots is inferred by the receiverfrom the number of packets it receives, as well as the number ofpossible collisions. The receiver passes this information to thetransmitter (e.g., transmitter 10 in FIG. 1). If the transmitter seesmany collisions, it increases the number of slots. If the number ofcollisions fall below a predefined number, the transmitter decreases thenumber of slots. To avoid collisions from other users on the samechannel, the transmitter, such as a transmitter 10 shown in FIG. 1, usescarrier sensing before sending messages to the tags. Each round can havean arbitrary amount of delay before the next. This ensures that thebackscatter system does not hog the channel. The use of rounds allowsfor fairness between the backscatter system and other users of thechannel. The use of slots within the backscatter system allows forfairness between tags.

FIG. 3 shows the durations of 30 million packets on channel 6 collectedin a lecture hall. In the bimodal distribution seen in FIG. 3, nearly78% of packets last less than 500 μs and nearly 18% of the packets last1500 μs-2700 μs. With a pulse-width error bound of 25 μs, theprobability of an ambient packet having the same length as packets inaccordance with one embodiment of the present invention is about 0.03%.

Transmitting Coordination Messages to Tags

In accordance with one aspect of the present invention, thecommunication from the transmitter to the tags is performed using atechnique that consumes relatively low power and dispenses with the needfor the tag to decode packets. To achieve this, in one embodiment, anenvelope detector is used to enable communication between thetransmitter and the tag. Low-power envelope detectors typically consumesless than 1 μW. Such an envelope detector is configured to measureparameters that can be easily measured and modulated at the transmitterusing, for example, commodity hardware, as described further below.

Packet Length Modulation

In accordance with one embodiment of the present invention, packetlength modulation (PLM) is used to establish communication from thetransmitter to the tags. Packet duration is relatively easy for thetransmitter to control, works well at a range of distances, and isrobust in the presence of ambient network traffic. In the PLM schemeused in accordance with one embodiment of the present invention, a 0 bitis represented by packets of duration L₀ and a 1 bit is represented bypacket of duration L₁. To control the length of the packet, thetransmitter sends packets of pre-defined durations L₀ and L₁. The taguses an envelope detector to identify the presence and duration of apacket. If a packet duration equals L₀ or L₁ (within a predefined errorrange) a bit is recorded to a data buffer. If a packet has a durationdifferent than L₀ or L₁ (taking into account the predefined range) thepacket is treated as noise and discarded, thereby enabling the bits tobe received successfully in the presence of other transmissions.

In one embodiment, a backscatter tag, in accordance with the presentinvention, operating using a WiFi 802.11 g/n standard operates atapproximately 500 bps, which is sufficient for operating the MAC layer.

To send the scheduling messages, the transmitter may generate dummypackets. Alternatively, the transmitter may buffer existing trafficbefore sending it to the network interface card (NIC), and then reorderor repacketize to form the sequence of L₀ and L₁s. Therefore, as long asthe network is busy, the backscatter messages imposes negligibleoverhead on the rest of the channel.

FIG. 4 shows the rate of decoding success as a function of distance. Theexperiment associated with FIG. 4 was performed in a long hallway insidean office building. For a reference voltage of 1.8v, the system is seenas being able to successfully decode the scheduling messages with over70% accuracy when the tag is less than 4 m away from the transmitter.The system is seen as successfully decoding the preamble with about 50%a distance of 50 m. Due to increased SNR, higher accuracy is possible atclose proximity by increasing the reference voltage in the comparator. Aprototype system was built using off-the-shelf commodity 802.11g/n WiFi,ZigBee, and Bluetooth transceivers and a custom made backscatter tag, asdescribed further below.

FIG. 5 is a high-level simplified block diagram of a backscatter tag100, in accordance with one embodiment of the present invention.Backscatter tag 100 is shown as including, in part, a multiplexer 110, acodeword translator 105, a phase modulator 115, a frequency modulator120, a frequency shifter 125, and an antenna 130. Depending on the typeof the data received and the communication protocol used, codewordtranslator uses the output of either phase modulator 115, or frequencymodulator 120. Phase modulator 115 modulates the phase of the receivedpackets and the frequency modulator modulates the frequency of thereceived packets, as described in detail above. The output ofmultiplexer 110 is frequency shifted by frequency shifter 125 beforebeing transmitted by antenna 130.

Hardware Platform

802.11 g/n Transceiver

An 802.11g/n receiver disposed in a MacBook Pro laptop with a BroadcomBCM43xx WiFi card that supports 802.11a/b/g/n/ac was used. The WiFi cardwas placed into a monitor mode to report packets that had incorrectchecksums. After receiving the packets, tcpdump (a well-known software)was used to parse the packets and extract the tag bits.

Also an Intel 5300 WiFi card on an Intel NUC was used as the standard802.11g/n OFDM transmitter, transmitting at 15 dBm. The firmware usedwas the one described in “Tool release: gathering 802.11 n traces withchannel state information provided”, authored by Daniel Halperin, WenjunHu, Anmol Sheth, and David Wetherall, ACM SIGCOMM Computer CommunicationReview 41, 1 (2011), 53-53.

ZigBee Transceiver:

A TI CC2650 radio (http://www.ti.com/product/CC2650) was used as theZigBee transceiver whose the transmission power was set to 5 dBm, whichis the maximum power allowed by this radio. The CC2650 radio developmentboard CC2650EM-71D supports two types of antennas: a PCB on-boardantenna and an antenna with an SMA interface. In the experiment, theVERT2450 antenna was used because it has a wider beam. It was mounted onthe SMA interface, as described in “Ettus Research. [n.d.]. VERT2450Antenna. https://www.ettus.com/product/details/VERT2450.

Bluetooth Transceiver:

A TI CC2541 radio (http://www.ti.com/product/CC2541) was used as theBluetooth transceiver. This radio transmits at 1 Mbps and 0 dBm usingFSK modulation with a frequency deviation of 250 kHz and a bandwidth of1 MHz. The modulation index used is 0.5±0.01.

Tag:

The tag used has two VERT2450 antennas: one for reception and one fortransmission https://www.ettus.com/product/details/VERT2450. Thereception antenna is coupled to an LT5534 envelope detector, whichmeasures when an incoming signal starts. A 0.35 us delay was measuredbetween the starting point of an excitation signal and the indicatorsignal from the envelope detector. In other words, 0.35 μs after thearrival of the excitation signal, the envelope detector notifies theprocessor that the excitation signal has begun. In the evaluations, theperformance does not degrade when experiencing a 0.35 μs delay.

The other antenna is controlled by an ADG902 RF switch, which decideswhen and how to backscatter the excitation signal. The codewordtranslation module is implemented in a low-power FPGA AGLN250. A powermanagement module was used on the tag which provides 1.5V and 3.3V tothe rest of the system. The source code of the tag platform is availableat https://github.com/pengyuzhang/FreeRider.

Implementation Challenges

Each radio comes with its own physical layer stack with a specific setof channel codes, interleaving techniques and scrambling algorithms, allof which can interfere with codeword translation and render itineffective. A description of how to enable codeword translation in thepresence of these challenges is provided below.

Challenges in Backscattering OFDM WiFi Signals

FIG. 6 shows various block diagrams of an 802.11 g/n transmission andreception blocks. There are three factors that could cause difficultywhen decoding a backscattered WiFi signal, namely the scrambler, theconvolutional channel encoder, and interleaving. The scrambler is a datawhitening engine where it takes the input data and XORs it with apseudorandom sequence. A scrambler ensures that the data transmitted isnot all zeros nor all ones, which causes a bad peak-to-average ratio.The channel encoder uses convolutional encoding to improve itsrobustness over wireless transmission. The interleaving engine re-ordersthe transmitted bits sequence to ensure that even a bursty error onwireless channel does not cause a burst of continuous errors on thereceived data. These three modules are described below because they areplaced before the modulator in an 802.11g/n transmitter. An explanationas to why such placement could cause unsuccessful backscatter decodingis also described below.

For any input sequence b₀, the transmitted signal S(t) can be formulatedas S(t)=f(b₀, b₁, . . . , b_(t)) where f( ) represents the operationsintroduced by the scrambler, channel encoder, interleaver, andmodulator. Since the corresponding demodulator, de interleaver, channeldecoder, and descrambler in the receiver provide the reverse operationsf⁻¹( ) the receiver is able to decode and output the transmittedsequence.

However, when a tag is present and produces a signal g(b₀, b₁, . . . ,b_(t)) using tag bits t₀, t₁, . . . , t_(n), the backscattered signalB(t) becomes the time-domain product between the tag signal and theexcitation signal. To help understand this further, the binary caseshown in equation 7 below is explained. The signal does not look like itis generated by XORing the excitation signal bits with the tag bits andpassed through f( ). Therefore, decoding the tag bits becomes hard.

B(t)=S(t)T(t)

=f(b ₀ ,b ₁ , . . . ,b _(n))×g(t ₀ ,t ₁ , . . . ,t _(n))

!=f(b ₀ ⊕t ₀ ,b ₁ ⊕t ₁ , . . . ,b _(n) ⊕t _(n))  (7)

A possible solution to this problem is redundancy, i.e., map one tag bitto multiple 802.11g/n bits. Instead of directly transmitting t₀, t₁, . .. , t_(n), the tag transmits a sequence where a tag repeats each bitmultiple times before switching to the next one. The following is adescription of the reason why such redundancy helps solve the problem.

The interleaving module is configured to interleave the data assigned toeach subcarrier. Interleaving is done per OFDM symbol. In other words,the interleaving module does not interleave data belonging to two OFDMsymbols. Therefore, as long as the tag bit duration is longer than anOFDM symbol, the interleaving module will not cause problems.

Both the scramble and channel encoder modules generate and maintain adeterministic structure of the data delivered to the modulator. Thescrambler uses the structure shown in FIG. 7 to do data whitening. Evenwhen the input is all zeros, the actual data transmitted is a non-zerosequence. Such data whitening reduces the peak-to-average power ratio inthe RF front end. The mathematical expression of the scrambler is shownin equation 8:

C[k]=b[k]⊕b[k−3]⊕b[k−7]  (8)

The channel encoder uses equation 9, shown below, to encode the data at6 Mbps where b(k) is the input bit C₁(k) and C₂(k) are the codewordsgenerated using a ½ coding rate.

C ₁[k]=b[k]⊕b[k−2]⊕b[k−3]⊕b[b−5]⊕b[k−6]

C ₂[k]=b[k]⊕b[k−1]⊕b[k−2]⊕b[b−3]⊕b[k−6]  (9)

For other bit rates, the channel encoder is different. The data injectedby the tag may corrupt the structures created by the two modules andmake backscatter decoding difficult. To overcome these challenges, thetwo modules were simulated using Matlab and it was found that as long asa tag injects one bit tag data on four OFDM symbols (96 WiFi bits in 6Mbps data rate), an error bit rate of nearly 1e⁻³ may be ontainted. Thisis because there is a one-to-one mapping between the input sequence b(k)and the output of the two modules C(k) or {C₁ [k], C₂ [k]}.

Equation 8 and equation 9 show that the sequence of {b[k]⊕1, b[k−1]⊕1, .. . b[k−7]⊕1} can generate C[k]⊕1 and {C₁[k]⊕1, C₂ [k]⊕1}. Therefore,when the tag does codeword translation and converts C(k) and {C₁[k],C₂[k]} to C[k]⊕1 and {C₁[k]⊕1, C₂[k]⊕1}, the corresponding modules atthe receiver should output {b[k]⊕1, b[k−1]⊕1, . . . , b[k−7]⊕1}. Thisresult has been proven using empirical Matlab simulation and real systemimplementation with a MacBook Pro laptop as the backscatter decoder.

The last factor that may impact backscatter decoding is the pilot tone.Pilot tones in an OFDM symbol are used for correcting the phase error.Such phase error correction could remove the additional phase offsetintroduced by a tag, and render incorrectly decoded tag data. However,there are a number of WiFi chips, such as Broadcom's BCM43xx, that donot use pilot tones for phase error correction and are this able tocorrectly decode the backscattered tag data.

Challenges in Backscattering ZigBee

ZigBee uses OQPSK modulation where there is a constant time-domainoffset (half a bit) between the in-phase signal and the quadraturesignal. Such offset is introduced for reducing the signalPeak-to-Average Power Ratio (PAPR) by avoiding the 18 phase transitionbetween neighboring bits. If the tag introduces a 180° phase transitionbetween neighboring bits in the backscattered ZigBee, it may corrupt theOQPSK signal structure and cause trouble decoding.

One solution to this problem is embedding one tag bit to each multiple(N) OQPSK symbols. When a tag transmits data one, instead of introducingthe 180° phase offset on a sine OQPSK symbol, the tag introduces thesame 180° additional phase offset on N OQPSK symbols. The firsttag-modified OQPSK symbol might be incorrectly decoded by a commercialZigBee decoder because of the potential OQPSK signal structure violationdescribed above. However, the following N−1 tag-modified OQPSK symbolscan be correctly decoded because the structure of OQPSK signals ismaintained. Therefore, as long as a relatively large N is selected, thedata information may be embedded in ZigBee traffic. In one example, Nwas selected to have a value of 8.

Challenges in Backscattering Bluetooth

There are two challenges to overcome in decoding a backscatteredBluetooth signal, namely modulation index i, and channel bandwidth w.Modulation index i is defined as

$\frac{f_{1} - f_{0}}{w}$

and represents the ratio between the frequency deviation of an FSKsignal and the bandwidth it occupies. A commercial Bluetooth radiousually uses a modulation index 0.5. When a tag, in accordance withembodiments of the present invention, toggles its RF transistor at Δf,while generating the desired backscattered signal, the tag alsogenerates an undesired signal on the other side of the spectrum as shownin FIG. 8. This undesired signal is generated because the backscatteredsignal is the time-domain product between the Bluetooth signal and thetag signal. Therefore a double-sideband backscatter signal is generated.

In accordance with one embodiment of the present invention, theundesired backscattered signal is eliminated by taking advantage of thefact that a Bluetooth radio treats signals outside of a channel asinterference and is able to eliminate them. Therefore, the selection ofΔf needs to satisfy the following two conditions to ensure that theundesired signal remains outside of the backscatter channel and is thuseliminated:

$\begin{matrix}\left\{ \begin{matrix}{{f_{1} + {\Delta \; f}} > {f_{1} + {\left( {1 - i} \right)\frac{w}{2}}}} & {{FSK}\mspace{14mu} {radio}\mspace{14mu} {data}\mspace{14mu} {one}} \\{{f_{0} - {\Delta \; f}} < {f_{0} - {\left( {1 - i} \right)\frac{w}{2}}}} & {{FSK}\mspace{14mu} {radio}\mspace{14mu} {data}\mspace{14mu} {zero}}\end{matrix} \right. & (10)\end{matrix}$

Low-Power Tag Design

To achieve low power consumption, a tag, in accordance with embodimentsof the present invention uses a ring oscillator to generate the squarewave signals needed for achieving the frequency shifting. One suchdesign is described in the article “Enabling practical backscattercommunication for on-body sensors”, authored by Pengyu Zhang, MohammadRostami, Pan Hu, and Deepak Ganesan, proceedings of the conference onACM SIGCOMM 2016, pp. 370-383. In one specific prototype formed using a65 nm technology node, the overall power consumption of the tag isnearly 30 μW depending on the excitation signal. Most of the power(e.g., 19 μW) is consumed by generating the 20 MHz clock needed forfrequency shifting. It was determined that 12 μW was used for operatingthe RF switch and 1-3 μW was used for running the control logic whichdetermines the type of the codeword translator to run.

Experimental Setup

FIGS. 9A and 9B respectively show experiments for testing a tag'sperformance when the tag is deployed in a line-of-sight (LOS) andnon-line-of sight (NLOS) setup. The tag was positioned 1 m away from thetransmitter (802.11g/n WiFi, ZigBee, or Bluetooth). No hardwaremodifications were performed on the commodity radio transmitters. Thereceiver was then moved away from the tag as the tag's throughput, biterror rate (BER), and received signal strength indicator (RSSI) weremeasured. In the LOS experiments, all devices are placed in a hallway.In the NLOS experiments, the transmitter and the tag are deployed in aroom while the receiver is deployed in a hallway. In the NLOSdeployment, the backscattered signal passes through multiple walls.

Tag's Backscatter Performance with 802.11g/n WiFi Deployed in LOS

FIG. 10A shows the throughput of a tag, in accordance with oneembodiment of the present invention, as a function of distance betweenthe tag and the receiver in a LOS deployment. The 802.11g/n WiFitransmitter sends its OFDM signal at 11 dBm. It is seen that thereceiver remains able to decode the backscattered signal at a distanceof 42 m. This distance is 1.4 times longer than the maximum distancereported in “Passive WiFi: bringing low power to Wi-Fi transmissions”,authored by Bryce Kellogg, Vamsi Talla, Shyamnath Gollakota, and JoshuaR Smith, 2016, 13th USENIX Symposium on Networked Systems Design andImplementation (NSDI 16) 151-164; and 8.4 times longer than the maximumdistance achieved by FS-Backscatter, as reported in “Enabling practicalbackscatter communication for on-body sensors”, authored by PengyuZhang, Mohammad Rostami, Pan Hu, and Deepak Ganesan, Proceedings of the2016 conference on ACM SIGCOMM. Such long communication distance issufficient for many Internet-of-Things applications.

A tag, in accordance with one embodiment of the present invention,achieves nearly 60 kbps data rate when the receiver is less than 18 maway from the tag. When the receiver moves farther within nearly 26 m-36m from the tag, the throughput decreases to nearly 15 kbps. This is arelatively lower data rate because OFDM symbols are longer in durationthan DSSS symbols. It is seen that the bit error rate remains low evenat longer distances as shown in FIG. 10B despite the fact that RSSI doesdegrade across distance as shown in FIG. 10C. For example, a bit errorrate of 1e⁻³ is achieved when the receiver is positioned 40 m away fromthe tag. Accordingly, at longer distances, if a backscattered packetreaches the receiver, then it is very likely that the tag will be ableto extract the bits with low BER. If the header itself is not decoded,then packet loss increases and throughput is degraded.

Tag's Backscatter Performance with 802.11g/n WiFi Deployed in NLOS

In the NLOS experiment, the 802.11g/n transmitter and the tag wereplaced in a room while the receiver was moved away in a hallway. FIG.11A shows the throughput of the system in this setup. The receiver isable to receive the backscattered packets when it is 22 m away from thetag Similar to the LOS deployment, a data rate of nearly 60 kbps isachieved when the receiver is less than 14 m away from the tag. Atlonger distances, the backscatter throughput degrades to nearly 20 kbps.FIG. 11B shows the BER of the system in the NLOS deployment. Similar tothe LOS deployment, a low BER is achieved across various distances.However, backscatter communication appears to stop at 22 m even thoughan RSSI of −84 dBm is achieved at 22 m as shown in FIG. 11C. It is seenthat when the receiver is more than 22 m away from the tag, thebackscattered signal needs to pass one more wall before reaching thereceiver as shown in FIG. 9B. As a result, the signal becomes too weakand the packet header is not detected.

Backscatter with ZigBee

FIG. 12A shows the throughput of a tag, in accordance with oneembodiment of the present invention, as a ZigBee receiver moves awayfrom the tag. The receiver receives backscattered packets from nearly 22m away. FIG. 12B shows the BER (bit error rate) of a tag, in accordancewith one embodiment of the present invention, as a ZigBee receiver movesaway from the tag. FIG. 12C shows that the received signal strengthdegrades to −97 dBm at 22 m, which is close to the noise floor of theZigBee radio. Therefore, receiving the backscattered packets at longerdistances becomes challenging. A backscatter data rate of nearly 14 kbpsis achieved when the receiver is less than 12 m away from the tag. Atfarther distances, the throughput degradation is not severe. A data rateof 12 kbps is achieved at a distance of 20 m. The bit error rateachieved is nearly 5e⁻² across all distances, higher than the case whenthe excitation signal is 802.11g/n.

Backscatter with Bluetooth

FIG. 13A shows the throughput of a tag, in accordance with oneembodiment of the present invention, as a Bluetooth receiver moves awayfrom the tag. It is seen that the receiver decodes backscatter packetsup to 12 m. FIG. 13B shows the BER of a tag, in accordance with oneembodiment of the present invention, as a Bluetooth receiver moves awayfrom the tag. FIG. 13C shows that the backscatter signal has a strengthof nearly −100 dBm at 12 m, close to the noise floor. Therefore,decoding the backscattered packets at farther distance becomeschallenging. When the receiver is less than 10 m away from the tag, adata rate of nearly 50 kbps is achieved. Throughput degrades to 19 kbpsat 12 m and the BER increases to 0.23.

Impact of Distance Between Transmitter and Tag

To measure this effect, the distance between the tag and the transmitterwas varied up to a point where backscatter communication could besustained. FIG. 14 shows the result of this experiment. Whenbackscattering 802.11g/n signal, at a transmitter-to-tag distance of 4m, the maximum receiver-to-tag distance is measured as 8 m. This is lessthan the 42 m achievable when the transmitter-to-tag distance is 1 m.Decreasing the receiver-to-tag communication distance yields a slightincrease in the achievable transmitter-to-tag distance. The operationalregime of the tag system is shown in the area 210 of FIG. 14.

When a ZigBee or Bluetooth radio is used, both the transmitter-to-tagdistance and the receiver-to-tag distance become shorter. The maximumtransmitter-to-tag distance is 2 m and 1.5 m for ZigBee and Bluetoothradios respectively, and the corresponding operational regime of tagsystem is marked with 220 and 230 respectively. Both regimes are smallerthan when an 802.11g/n signal is used primarily because the transmissionpower of the ZigBee and Bluetooth radios is smaller (5 dBm and 0 dBm vs15 dBm).

Co-Existence with WiFi Networks

To determine if a tag, in accordance with embodiments of the presentinvention, can co-exist with existing WiFi networks, WiFi traffic wasgenerated in which a laptop transfers files via WiFi on channel 6 (2.437GHz). Then, a backscatter was run on nearly 2.472-2.48 GHz (the exactfrequency depends on the type of the excitation signal). A measurementwas made to determine how the WiFi traffic and backscatter impact eachother when the backscatter channel does not conflict with the WiFichannel.

Does Backscatter Impact WiFi

FIG. 15 shows the WiFi throughput when the backscatter tag is eitherpresent or absent. When backscatter is absent, WiFi is able to transmitat nearly 37.4 Mbps median data rate. Then, a backscatter tag is placed1 m away from the WiFi receiver and the WiFi throughput is measured. Thetag runs three codeword translators sequentially, one for backscattering802.11g/n WiFi, one for ZigBee, and one for Bluetooth. The median WiFithroughput measured is 37 Mbps, 37.9 Mbps, and 36.8 Mbps respectively,close to the WiFi throughput when the backscatter tag is not present.Therefore, a backscatter tag does not cause interference on an existingWiFi traffic.

Does WiFi Impact Backscatter

To determine whether or not concurrent WiFi traffic impacts backscatterdecoding, the following experiment was performed. When a tagbackscatters its data in a channel that is occupied by WiFi traffic,backscatter throughput degrades to zero because the WiFi traffic isusually approximately 30 dB higher than the backscattered signal.Therefore, backscatter suffers. Therefore, an experiment is performedunder a condition where an existing WiFi traffic does not share the samechannel as backscatter, so as to understand how backscatter performs inthe presence of WiFi traffic on adjacent channels.

FIG. 16(A) shows the backscatter throughput when an 802.11g/n WiFi isused as the excitation signal with the tag backscattering on channel 13(2.472 GHz), and the WiFi traffic running on channel 6 (2.437 GHz). Whenthe WiFi traffic is absent, 61.8 kbps median backscatter throughput isachieved. When the WiFi traffic is present, the backscatter throughputremains at 61.8 kbps. However, it is seen that the backscatter is ableto reach 68 kbps for 20% of the time when the WiFi traffic is absent,and degrades to 35 kbps for 10% of the time when the WiFi traffic ispresent. Therefore, the presence of WiFi traffic impacts the backscatterthroughput. To minimize this impact, technique similar to the onedescribed in “Hitchhike: Practical backscatter using commodity WiFi”,authored by Pengyu Zhang, Dinesh Bharadia, Kiran Joshi, and SachinKatti, 2016 ACM SENSYS, may be used together with RTS-CTS to reserve thechannel for backscatter.

FIGS. 16(B) and 16(c) show the backscatter throughput when a tagbackscatters a ZigBee signal and a Bluetooth signal respectively. Inboth experiments, the tag backscatters on channel 2.48 GHz. It is seenthat the backscatter throughput difference between WiFi traffic beingpresent or not is only approximately between 1-2 kbps. Therefore, anexisting WiFi traffic does not impact the backscatter performance when aZigBee or Bluetooth radio is leveraged. One reason is that both suchradios are narrowband, and therefore, have better performance infiltering out-of-band interference.

Evaluating MAC Layer Performance

The performance of the system when communicating with multiple tags isalso studied. FIG. 17(a) shows the aggregated throughput whenrespectively 4, 8, 12, 16, and 20 tags are positioned in the path of thetransmitter. The aggregate throughput is lower than that of a single tagfor two reasons: control overhead and collisions. As the number of tagsincreases, the aggregated throughput increases. This is due to therelative ratio of control overhead decreasing when more transmissionslots are used. If the simulation is extended beyond the 20 tags, thethroughput asymptotically approaches to at about 18 kbps. If there areno collisions (i.e. a TDM scheme), the simulation throughputasymptotically approaches to about 40 kbps.

Framed Slotted Aloha is well-suited to applications that have low dataneeds and where the number of active tags can increase or decreasewithout warning, such as inventory tracking. More data-intensiveapplications would benefit from a time division scheme, which would bepossible to implement in a tag, in accordance with embodiments of thepresent invention. The analysis above was limited to a single MAC layerdesign.

FIG. 17(b) shows the Jain's fairness index, as described in “Throughputfairness index: An explanation”, Technical Report, Department of CIS,The Ohio State University authored by Raj Jain, Arjan Durresi, and GojkoBabic, 1999, when 4, 8, 12, 16, and 20 tags are present. When the numberof tags increases, the fairness index stays about the same because thescheduler dynamically allocates a larger number of slots when more tagsare present. The averaged fairness index is 0.85 when 20 tags arepresent, suggesting that most of tags still obtain similar opportunitiesfor data transmission.

The above descriptions of embodiments of the present invention areillustrative and not limitative. For example, the various embodiments ofthe present inventions are not limited by the communication protocol,802.11 g/n, Bluetooth, ZigBee or otherwise, used for signaltransmission. Other modifications and variations will be apparent tothose skilled in the art and are intended to fall within the scope ofthe appended claims.

What is claimed is:
 1. A backscatter tag communicate device comprising:a receiver configured to receive a packet conforming to a communicationprotocol defining a plurality of codewords and characterized by a firstfrequency; a codeword translator configured to translate each of atleast a first subset of the plurality of codewords disposed in thepacket to another one of the plurality of codewords defined by theprotocol in response to a data that the backscatter tag is invoked totransmit; and a transmitter configured to transmit the packet suppliedby the codeword translator at a second frequency different than thefirst frequency.
 2. The backscatter tag communication device of claim 1wherein the translation of a codeword is achieved by changing a phase ofthe codeword.
 3. The backscatter tag communication device of claim 2wherein said communications protocol is one of the ZigBee or WiFi802.11(g) or WiFi 802.11(n) communications protocol.
 4. The backscattertag communication device of claim 1 wherein the translation of acodeword is achieved by changing a frequency of the codeword.
 5. Thebackscatter tag communication device of claim 4 wherein saidcommunications protocol is the Bluetooth communications protocol.
 6. Thebackscatter tag communication device of claim 2 wherein said phasechange is defined by a data rate of the transmitter.
 7. The backscattertag communication device of claim 1 wherein said backscatter tagcommunication device transmits the packet supplied by the codewordtranslator during one of a plurality of time slots at random.
 8. Thebackscatter tag communication device of claim 7 wherein said pluralityof time slots is a variable number.
 9. The backscatter tag communicationdevice of claim 1 wherein said received packet has a duration selectedfrom one of a plurality of predefined values.
 10. The backscatter tagcommunication device of claim 9 wherein said backscatter tagcommunication device further comprises an envelope detector to detectthe duration of the received packet.
 11. A method of communication via abackscatter tag, the method comprising: receiving a packet conforming toa communication protocol defining a plurality of codewords andcharacterized by a first frequency; translating each of at least a firstsubset of the plurality of codewords disposed in the packet to anotherone of the plurality of codewords defined by the protocol in response toa data that the backscatter tag is invoked to transmit; and transmittingthe packet supplied by the codeword translator at a second frequencydifferent than the first frequency.
 12. The method of claim 11 whereinthe translation of a codeword is achieved by changing a phase of thecodeword.
 13. The method of claim 12 wherein said communicationsprotocol is one of the ZigBee or WiFi 802.11(g) or WiFi 802.11(n)communications protocol.
 14. The method of claim 11 further comprising:translating a codeword by changing a frequency of the codeword.
 15. Themethod of claim 14 wherein said communications protocol is the Bluetoothcommunications protocol.
 16. The method of claim 12 wherein said phasechange is defined by a data rate of a transmitter transmitting thepacket.
 17. The method of claim 1 further comprising: transmitting thetranslated packet during one of a plurality of time slots at random. 18.The method of claim 17 wherein said plurality of time slots is avariable number.
 19. The method of claim 1 wherein said received packethas a duration selected from one of a plurality of predefined values.20. The method of claim 19 wherein said backscatter tag comprises anenvelope detector configured to detect the duration of the receivedpacket